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Radiation

The radiative transfer code is adapted from the one developed in the context of terrestrial modeling by Fouquart and Bonnel (1980)  for solar radiation and Morcrette et al.  (1986) for thermal radiation. This code, originally developed for the climate GCM of Laboratoire de Météorologie Dynamique, has since been included by Morcrette in the operational model of the European Center for Medium-Range Weather Forecasts (ECMWF).
Recently, at LMD, in the frame of extension to ESA Contract 11369/95/NL/JG, the infrared part of the radiative transfer code has been rewritten with a net exchange formulation.

For thermal radiation,

effects of both atmospheric carbon dioxide and dust are included. Particular attention was given to the parameterization of absorption by the tex2html_wrap_inline5470 m band with inclusion of Doppler effect [3]. Cooling rates computations were carefully validated by comparison to line-by-line integrations. The model, in which Doppler effects are introduced, is very accurate up to 70 km. The thermal spectrum is divided into three parts, one for the core of the tex2html_wrap_inline5470 m band, one for the wings and the third one for the remaining part of the spectrum. For the three parts, the transmission by dust is computed using grey absorption approximation. For the two intervals of the tex2html_wrap_inline5470 m band, the total transmissivity is evaluated as the product between transmissivity of dust and that of carbon dioxide. Strictly speaking, it can be shown that this evaluation of the combined transmissivity is valid when there is no correlation between the spectral variations of the two absorber. This is generally assumed for dust and carbon dioxide in the tex2html_wrap_inline5470 m band. Scattering is not taken into account because of the strong isotropy of the thermal radiation.

New model radiation scheme.

This new code is based on a net exchange formulation. The original Morcrette radiative code computes directly fluxes at each level. In the new approach, we first compute individual exchanges for each pair of layers, and then the budget for each layer. The net exchange between two layers is the product of two terms:
1) the difference between Planck functions, which depends on temperature profile, 2) the exchange coefficient, which only depends on optical properties.

The main interest of this new method is first to allow analysis of the relative importance of the various terms included in the budget (in the flux formulation, we just have a global value for the exchange of the considered layer with all the rest of the atmosphere), and second to save CPU time. Indeed, we can compute less often the optical coefficients since they vary slowly in time. The Planck function, which varies more quickly, can easily be computed more often, because it is just a polynomial approximation. This new method is less costly and more accurate.
For details, report to ESA Contract 11369/95/NL/JG (Work Package VI: Technical Report - New model radiation schemes, parameterization and validation).

Solar radiation:

in the original code developed by Fouquart and Bonel (1980), the upward and downward fluxes are obtained from the reflectance and transmittance of the layers. The interaction between gaseous absorption and scattering (by dust, molecules or clouds) is introduced using the photon path distribution method. At this stage, only absorption and scattering by dust (already present in the version of the code used at the ECMWF) is included in the Martian version although absorption by the near infra-red bands of carbon dioxide may become non negligible for very non-dusty conditions. The transmittance and reflectance of the layers are computed using the Delta-approximation to account for the strong asymmetry of the aerosol phase function.

The atmospheric dust content

is specified as a mixing ratio constant in both time and space, except for the vertical distribution which is taken according to Pollack et al.  (1990). Beyond simplicity, the reason for this choice is that dust transport has not been included in the GCM until now. This strong approximation may affect the global tex2html_wrap_inline5478 condensation-sublimation cycle which primarily depends on the local dust-content at low latitudes [13]. The optical parameters of dust (such as single scattering albedo and asymmetry factor) are taken from Pollack et al.  (1979) .


next up previous contents
Next: Vertical turbulent mixing Up: Les paramétrisation physique du Previous: Les paramétrisation physique du

HOURDIN Christophe
mardi, 9 novembre 1999, 17:34:37 MET